X-ray brightness control system with means to vary the power applied to the x-ray source



Oct. 3, 1967 R. B. NICHOLSON ETAL. 3,345,516

X-RAY BRIGHTNESS CONTROL SYSTEM WITH MEANS TO VARY THE POWER APPLIED TO THE X-RAY SOURCE FiledDec. 29, 1964 Walter S. Richardson BY xwg mpg United States Patent. ()fiice 3,345,515 Patented Get. 3, 1967 3,345,516 X-RAY BRIGHTNESS CGNTROL fiYSTEM WITH MEANS TO VARY THE POWER APPLIED TO THE X-RAY SOURCE Raymond B. Nicholson and Walter S. Richardson, Houston, Tern, assignors to Westinghouse Electric Corporation, East Pittshurgh, Pa, a corporation of Pennsylvania Filed Dec. 29, 1964, Ser. No. 421,857 6 Claims. (Cl. 250-103) ABSTRACT OF THE DISCLOSURE Power supply from an alternating current source to an X-ray tube causing illumination of a fluorescent screen is controlled by a pair of semiconductive controlled rectifiers which are alternately turned on at a time during successive A.C. half-cycle periods which is delayed according to preponderance in voltage in a secondary winding of one transformer means having a primary winding energized according to light intensity on the screen over the voltage in a secondary winding of another transformer means having a primary winding energized by the alternating current source. The two secondary windings being connected in series via a respective diode means to the gate electrode of the respective serniconductive controlled rectifier controlled thereby.

This invention relates to control systems for X-ray apparatus, and more particularly to a system for controlling the power supplied to an X-ray tube as a function of the brightness of a visible image produced by the X-rays on a fluorescent screen.

While not necessarily limited thereto, the present in Vention is particularly adapted for use in the art of cineradiography wherein motion pictures are taken of a fluorescent screen excited by X-rays passing through the body of a patient. For good photographicreproduction of the fluorescent screen, it is desirable that the brightness of the screen be held at a nearly constant level regardless of the thickness of the patient, or part of the patient being X-rayed.

The difference inpatient density is normally so great that merely varying the quantity of emitted X-rays by adjusting the current flowing through the heating element of the" tube will not give a sufiicient range of light change tocornpensate for changes in density. Consequently, it becomes necessary to vary the power input to the X-ray tube as a function of changes in thelight intensity of the screen.

1 Accordingly, as an overall object, the present invention seeks to provide new and improved apparatus for stabilizing the brightness of an image produced with 'X-rays on a fluorescent screen by varying the power input to an X-ray tube.

Another object of the invention is to provide apparatus for controlling the power input to an X-ray tube by means of a phase control system employing semiconductive controlled rectifiers.

Still another object of the invention is to provide a phase controlled power supply system for an X-ray tube wherein firing of semiconductive controlled rectifiers is achieved in a novel manner by the use of differential transformer techniques.

In accordance with the invention, the usual alternating current power supply for the X-ray tube is applied to a conventional rectifier circuit through circuitry including at least two semiconductive controlled rectifiers arranged such that one rectifier will conduct during one half cycle of the alternating current power supply while the other rectifier will conduct during the other half cycle. By

controlling the point during each half cycle at which the respective semiconductive controlled rectifiers conduct, the power supplied to the X-ray tube may be varied, thereby also varying the brightness of the image produced on a fluorescent screen onto which the X-rays impinge.

The point at which the respective semiconductive controlled rectifiers con-duct during each half cycle is controlled in accordance with the invention by means of apparatus including a photosensitive device adapted to produce a control signal which varies in magnitude as a function of the light intensity of the image produced on the fluorescent screen. By utilizing this control signal to vary the point at which the semiconductive controlled recti fiers conduct during alternate half cycles of the alternating current power supply, a servo system is established in which, as the light intensity rises, the power supplied to the X-ray tube decreases to compensate for the rise, and vice versa.

Preferably, the means for controlling power supplied to the X-ray tube as a function of light intensity at the fluorescent screen comprises a differential transformer arrangement wherein a first voltage tending to fire a semiconductive controlled rectifier during'each half cycle is opposed by a second voltage which varies in magnitude as a function of the intensity of the image produced on the fluorescent screen. Thus, as the light intensity of the image increases, the magnitude of the second voltage will increase also to cause an associated semiconductive controlled rectifier to fire later during a half cycle of the power supply, thereby decreasing the power to the Xray tube as well as the light intensity. Conversely, when the light intensity decreases, the magnitude of the second voltage decreases to increase the power to the X-ray tube as well as the light intensity of the image.

The above and other objects and features of the invention will become apparent from the following detailed description taken in connection with the accompanying drawings which form a part of this specification, and in which:

FIGURE 1 is a schematic circuit diagram of the control system of the invention; and

FIG. 2 comprises waveforms illustrating the operation of the circuit of FIG. 1.

With reference now to the drawings, and particularly to FIG. 1, a patient 10 is shown resting on a table 12 through which X-rays from tube 14 pass. After passing through the table 12 and the patient 10, the X-rays are directed onto a conventional fluorescent screen 16 adapted to produce the usual visual X-ray image of light and dark areas. Above the screen 16 is almotion picture camera 18 which continually photographs the image produced on screen 16.

. The X-ray tube 14- is conected, as shown, to the usual rectifier 20 supplied with power from a conventional alternating current supply and control circuit 22. The current flowing through the filament 24 of the X-ray tube 14 may be controlled by means of a filament control circuit 26 in accordance with usual practice; and it will be understood that as the current through the filament 24 is increased, the heat generated by the filament is also increased to increase the magnitude of the X-rays delivered through the body of the patient 10.

As was mentioned above, for good photographic reproduction of the fluorescent screen 16 by camera 18, it is necessary that the brightness of the screenbe held at a nearly constant level regardless of the thickness of the patient or part of the patient being photographed. Furthermore, the difference in patient density is to great that merely varying the quantity of the X-rays by varying the current through the filament 24 will not give a sulficient range of light change to compensate for variations 1.3 in brightness on screen 16. Accordingly, it becomes necessary to vary the power supplied to the X-ray tube 14.

in accordance with the present invention, the light intensity emitted by screen 16 is controlled by means of a pair of semiconductive controlled rectifiers, specifically silicon controlled rectifiers 28 and 30, connected in parallel, back-to-back relationship between one terminal of rectifier 2d and one of the output terminals of the power supply 22. As is well known to those skilled in the art, the silicon controlled rectifiers 28 and 30 are the equivalents of thyratrons. Each rectifier includes an anode 32, cathode 34 and a gate electrode 36. Each silicon controlled rectifier acts as a two-terminal switch and will block curent How in either direction until a critical breakover voltage is exceeded or until a positive voltage is applied to its gate electrode 36 with respect to its cathode. By applying pulses to the gate electrode 36 during the positive and negative half cycles of the alternating current voltage source 22 in delayed time relationship with respect to the zero crossings of the alternating current waveform, a portion of the power from supply 22 can be applied to the rectifier 2t and X-ray tube 14, depending upon the time delay.

The circuitry for controlling the silicon controlled rectifiers 2-8 and 30 includes a pair of transformers 38 and 4t Transformer 40 is provided, as shown, with a primary winding 42 connected across the output terminals of the power supply 22. Inductively coupled to the core of transformer 40 is a center-tapped secondary winding 44 having its opposite ends connected through diodes 46 and 48 to one end of a bias winding 50 on transformer 38. The center tap of winding 44 is connected through a potentiorneter 52 to the other end of bias winding 50; and it will be appreciated that the combination just described comprises a full-wave rectifier in which the rectified voltage is applied across the bias winding 50. The magnitude of this rectified voltage, as applied across winding 50, is controlled by the position of the tap on potentiometer 52.

Inductively coupled to the core of transformer 38 is a control winding 54 connected through amplifier 56 to a photocell 58. The photocell 58 is subjected to the intensity of the light emitted by the fluorescent screen 16 such that it will produce an electrical signal having a magnitude directly proportional to the light intensity. This electrical signal, after amplification in amplifier 56, is applied to the control winding 54 as shown.

As will be understood, the photocell 58 is illustrative of only one type of photosensitive device which may be utilized in accordance with the present invention for the purpose of converting the light intensity emitted by screen 16 into an electrical signal. Thus, the photocell 58 may be replaced by a photomultiplier tube or, if desired, by light-sensitive resistors.

Referring again to silicon controlled rectifier 28, its gate electrode 36 is connected to its cathode 34 through resistor 6% Gate electrode 36 is also connected through diode 62 to one end of secondary winding 64 on transformer 38. The other end of the winding 64 is connected through winding 66 on transformer 40 to the cathode 34 of rectifier 28. In a similar manner, the gate electrode 36 of rectifier 30 is connected to its cathode through resistor 7t and to one end of secondary winding 72 on transformer 33 through diode 75. The other end of winding 72 is connected through winding 74 on transformer 46! to the cathode 34 of rectifier 30.

Points of instantaneous like polarity are indicated by dots on the various windings on transformers 38 and 40. As was explained above, a positive potential must be applied to gate electrode 36 with respect to the cathode 34 of rectifier 28 or 30 in order to render it conducting. Furthermore, since windings 66 and 74- are wound with opposite polarities, it will be appreciated that rectifier 28 can be rendered conductive during only one-half cycile of the input alternating current voltage while rectifier 30 can be rendered conductive during the other half cycle only. The point during each half cycle at which the rectifiers 28 and 39 fire is determined by the voltages across the bias and control windings 5t) and 54. That is, the voltage induced in winding 72, for example, will tend to bias diode 75 to cutoff and will oppose the voltage induced in winding 72 during one-half cycle of the alternating current power supply 22 tending to bias the diode 75 in the forward direction. Furthermore, the point at which the voltage induced in winding 74 overcomes that induced in winding 72 to bias diode 75 in the forward direction during a half cycle will be dependent upon the voltages induced in windings 50 and 54. The voltage in duced in winding 50, of course, is dependent upon the setting of potentiometer 52; while that induced in winding 54 is dependent upon the intensity of the light produced by the screen 16.

Operation of the invention may best be understood by reference to FIG. 2 wherein waveform A illustrates the output alternating current waveform of the power supply 22. During the first half cycle, a voltage will be induced in winding 66, for example, opposing that induced in winding 64 by the windings 50 and 54. That is, the two voltages are in subtractive relationship. However, at time t in the first half cycle, the voltage induced in winding 66 will overcome that induced in winding 64 to bias diode 62 in the forward direction and fire rectifier 28, whereupon the rectifier 28 will conduct to pass through rectifier 20 approximately one-half of the first half cycle of the alternating current voltage from source 22. This is shown by waveform B in FIG. 2.

During the first half cycle, the voltages on windings 72 and 74 are additive, maintaining the diode 75 cutoff during the entirety of the first half cycle. However, on the next half cycle, a voltage will be induced in winding 74 opposing that induced in winding 72. At time t in the second half cycle, the voltage induced across winding 74 will overcome that induced in winding 72 by an amount sufiicient to bias diode 75 in the forward direction, whereupon the rectifier 30 will conduct as shown by waveform B to pass to rectifier 20 approximately one-half of the negative half cycle of the alternating current voltage from source 22. During the second negative half cycle, the voltages on windings 64 and 66 are additive to maintain diode 62 cutoff.

Let us assume, now, that the light intensity produced by the screen 16 drops. Under these circumstances, the voltage at the output of amplifier 56 applied to winding 54 will also drop as will the voltages induced across windings 64- and 72. Now, when a voltage is induced across winding 66 during the first half cycle, it will overcome that induced in winding 64 earlier in the cycle due to the decreased voltage across winding 54. Consequently, rectifier 23 will now fire at time t during the positive half cycle. Similarly, rectifier 30 will now fire at time t during the negative half cycle. The result, of course, is that more power is supplied to the rectifier 20, and this increase in power increases the intensity of the X-ray beam to thereby also increase the light intensity produced by screen 16, whereupon the voltage across winding 54 is increased and the system stabilizes about the light intensity level established by the setting of potentiometer 52.

It will be appreciated that the system just described comprises a servo system wherein a decrease in light intensity from the screen 16 will result in a decrease in voltage across winding 54 and an increase in the power delivered to X-ray tube 14 to again raise the light intensity to the desired level. Conversely, if the light intensity produced by screen 16 should increase above that established by the setting of potentiometer 52, the voltage across winding 54 will also increase, thereby shortening the time during each half cycle that rectifier 28 or 30 conducts. This, in turn, results in a decrease of the light intensity from screen 16 to again stabilize the system.

Although the invention has been shown in connection with a certain specific embodiment, it will be readily apparent to those skilled in the art that various changes in form and arrangement of parts may be made to suit requirements Without departing from the spirit and scope of the invention.

We claim as our invention:

1. In an X-ray system of the type in which X-rays from an X-ray tube impinge upon a fluorescent screen to pro duce a visual image thereon, and wherein the X-ray tube is energized from an alternating current source through a rectifier circuit; the combination of means for maintaining the brightness of the image on the fluorescent screen at a substantially fixed level, comprising a photosensitive devioe for generating a control signal having a magnitude which varies as a function of the brightness of said image, means connecting said alternating current source to the rectifier including at least one semiconductive controlled rectifier having a cathode, anode and gate electrode and adapted to conduct current from the alternating current source to the rectifier circuit during a half cycle of the alternating current source, first transformer means having a primary winding connected to said alternating current source, second transformer means having a primary Winding connected to said photosensitive device so as to be energized by said control signal, a current path connecting the gate electrode of said semiconductive controlled rectifier to its cathode, said current path including a secondary winding on the first transformer means in series with a secondary winding on the second transformer means, the Voltages induced in said secondary windings being subtractive on one-half cycle of the alternating current source and additive on the other half cycle, and a diode in the current path connected such that it is biased to cutoff when the voltages induced in the secondary windings are additive, the diode being rendered conductive to fire said semiconductive controlled rectifier after the voltage induced in the secondary winding on the first transformer means exceeds that induced in the secondary winding on the second transformer means, the arrangement being such that the point in a half cycle at which the semiconductive controlled rectifier fires is dependent upon the magnitude of said control signal.

2. In an X-ray system of the type in which X-rays from an X-ray tube impinge upon a fluorescent screen to produce a visual image thereon, and wherein the X-ray tube is energized from an alternating current source through a rectifier circuit; the combination of means for maintaining the brightness of the image on the fluorescent screen at a substantially fixed level, comprising a photosensitive device for generating a control signal having a magnitude which varies as a function of the brightness of said image, means connecting said alternating current source to the rectifier circuit including at least one semiconductive controlled rectifier having a cathode, anode and gate electrode and adapted to conduct current from the alternating current source to the rectifier circuit during a half cycle of the alternating current source, first transformer means having a primary winding connected to said alternating current source, second transformer means having a primary winding connected to said photosensitive device so as to be energized by said control signal, a current path connecting the gate electrode of said semiconductive controlled rectifier to its cathode, said current path including a secondary winding on the first transformer means in series with a secondary winding on the second transformer means, the voltages induced in said secondary windings being subtractive on one-half cycle of the alternating current source and additive on the other half cycle, a diode in the current path connected such that it is biased to cutoif when the voltages induced in the secondary windings are additive, the diode being rendered conductive to fire said semiconductive controlled rectifier after the voltage induced in the secondary winding on the first transformer means exceeds that induced in the secondary winding on the second transformer means, rectifier means inductively coupled to said first transformer means, a bias winding on said second transformer means, and apparatus including a potentiometer for applying the rectified output of said rectifier means across the bias winding on said second transformer means whereby the point in a half cycle of said alternating current source at which the semiconductive controlled rectifier fires will be dependent upon the setting of said potentiometer and the magnitude of said control signal applied to the first-mentioned primary winding on the second transformer means.

3. The X-ray system of claim 2 wherein said diode has its cathode connected to the gate electrode of said semiconductive controlled rectifier and its anode connected to one end of the secondary winding on said second transformer means, and wherein a resistor is connected between the gate electrode and cathode of said semiconductive controlled rectifier.

4. The X-ray system of claim 2 wherein said rectifier means comprises a center-tapped secondary winding on said first transformer means, a pair of diodes connecting opposite ends of said last-mentioned secondary winding to one end of the bias winding on the second transformer means, and wherein said potentiometer connects the center tap on said secondary winding on the first transformer means to the other end of the bias winding.

5. In an X-ray system of the type in which X-rays from an X-ray tube impinge upon a fluorescent screen to produce a visual image thereon, and wherein the X-ray tube is energized from an alternating current source through a rectifier circuit; the combination of means for maintaining the brightness of the image on the fluorescent screen at a substantially fixed level, comprising a photosensitive device for generating a control signal having a magnitude which varies as a function of the brightness of said image, means connecting said alternating current source to the rectifier circuit including a first semiconductive controlled rectifier having a cathode, anode and gate electrode sir id adapted to conduct on one-half cycle of the alternating current source and a second semiconductive controlled rectifier having a cathode, anode and gate electrode and adapted to conduct on the other half cycle of the alternating current source, first transformer means having a primary winding connected to said alternating current source, second transformer means having a primary winding connected to said photosensitive device so as to be energized by said control signal, a first current path connecting the gate electrode of said first semiconductive controlled rectifier to its cathode, a second current path connecting the gate electrode of the second semiconductive controlled rectifier to its cathode, each of said first and second current paths including a secondary winding on the first transformer means in series with a secondary winding on the second transformer means, the voltages induced in said secondary windings in the first current path being subtractive on a first half cycle of the alternating current source and additive on the second half cycle, the voltages induced in said secondary windings in the second current path being additive on said first half cycle of the alternating current source and subtractive on said second half cycle, and a diode in each current path connected such that it is biased to cutoff when the voltages induced in the secondary windings of its associated current path are additive, the diode in each current path being rendered conductive to fire its associated semiconductive controlled rectifier after the voltage induced in the secondary winding on the first transformer means eX- ceeds that induced in the secondary winding on the second transformer means and its associated current path, the arrangement being such that the point in a half cycle at which each semiconductive controlled rectifier fires is dependent upon the magnitude of said control signal.

6. The X-ray system of claim 5 and including a bias winding on said second transformer means, and a source of voltage connected to said bias winding for establishing an initial flux level in the core of said second transformer means, which flux level is varied by the control current through said primary Winding on the second transformer means.

References Cited UNITED STATES PATENTS 2,962,594 11/1960 Duffy 250-95 5 RALPH G. NILSON, Primary Examiner.

ARCHIE R. BORCHELT, Examiner.

W. F. LINDQUIST, Assistant Examiner. 

1. IN AN X-RAY SYSTEM OF THE TYPE IN WHICH X-RAYS FROM AN X-RAY TUBE IMPINGE UPON A FLUORESCENT SCREEN TO PRODUCE A VISUAL IMAGE THEREON, AND WHEREIN THE X-RAY TUBE IS ENERGIZED FROM AN ALTERNATING CURRENT SOURCE THROUGH A RECTIFIER CIRCUIT; THE COMBINATION OF MEANS FOR MAINTAINING THE BRIGHTNESS OF THE IMAGE ON THE FLUORESCENT SCREEN AT A SUBSTANTIALLY FIXED LEVEL, COMPRISING A PHOTOSENSITIVE DEVICE FOR GENERATING A CONTROL SIGNAL HAVING A MAGNITUDE WHICH VARIES AS A FUNCTION OF THE BRIGHTNESS OF SAID IMAGE, MEANS CONNECTING SAID ALTERNATING CURRENT SOURCE TO THE RETIFIER INCLUDING AT LEAST ONE SEMICONDUCTIVE CONTROLLED RECTIFIER HAVING A CATHODE, ANODE AND GATE ELECTRODE AND ADAPTED TO CONDUCT CURRENT FROM THE ALTERNATING CURRENT SOURCE TO THE RECTFIER CIRCUIT DURING A HALF CYCLE OF THE ALTERNATING CURRENT SOURCE, FIRST TRANSFORMER MEANS HAVING A PRIMARY WINDING CONNECTED TO SAID ALTERNATING CURRENT SOURCE, SECOND TRANSFORMER MEANS HAVING A PRIMARY WINDING CONNECTED TO SAID PHOTOSENSITIVE DEVICE SO AS TO BE ENERGIZED BY SAID CONTROL SIGNAL, A CURRENT PATH CONNECTING THE GATE ELECTRODE OF SAID SEMICONDUCTIVE CONTROLLED RECTIFIER TO ITS CATHODE, SAID CURRENT PATH INCLUDING A SECONDARY WINDING ON THE FIRST TRANSFORMER MEANS IN SERIES WITH A SECONDARY WINDING ON THE SECOND TRANSFORMER MEANS, THE VOLTAGES INDUCED IN SAID SECONDARY WINDINGS BEING SUBTRACTIVE ON ONE-HALF CYLCLE OF THE ALTERNATING CURRENT SOURCE AND ADDITIVE IN THE OTHER HALF CYCLE, AND A DIODE IN THE CURRENT PATH CONNECTED SUCH THAT IT IS BIASED TO CUTOFF WHEN THE VOLTAGES INDUCED IN THE SECONDARY WINDINGS ARE ADDITIVE, THE DIODE BEING RENDERED CONDUCTIVE TO FIRE SAID SEMICONDUCTIVE CONTROLLED RECTIFIER AFTER THE VOLTAGE INDUCED IN THE SECONDARY WINDING ON THE FIRST TRANSFORMER MEANS EXCEEDS THAT INDUCED IN THE SECONDARY WINDING ON THE SECOND TRANSFORMER MEANS, THE ARRANGEMENT BEING SUCH THAT THE POINT IN A HALF CYCLE AT WHICH THE SEMICONDUCTIVE CONTROLLED RECTIFIER FIRES IS DEPENDENT UPON THE MAGNITUDE OF SAID CONTROL SIGNAL. 